In an information recording medium, such as a CD and a DVD, for example, as described in patent documents 1 and 2 or the like, there is also developed an information recording medium, such as an optical disc of a multilayer type or dual layer type or multiple layer type, in which a plurality of recording layers are laminated or stacked on the same substrate. Then, on an information recording apparatus, such as a CD recorder, for performing the recording with respect to the dual layer type, i.e., two-layer type, optical disc, laser light for recording is focused or condensed on a recording layer located on the front as viewed from the irradiation side of the laser light (hereinafter referred to as an “L0 layer”, as occasion demands), to thereby record information into the L0 layer in an irreversible change recording method by heat or in a rewritable method. Moreover, the laser light is focused or condensed on a recording layer located on the rear of the L0 layer as viewed from the irradiation side of the laser light (hereinafter referred to as an “L1 layer”, as occasion demands), through the L0 layer or the like, to thereby record information into the L1 layer in the irreversible change recording method by heat or the rewritable method.
On the other hand, there is also disclosed a technology of performing the recording or reproduction in an “opposite method” or the like with respect to the L0 layer and the L1 layer. The “opposite method” herein is a recording or reproduction method in which the directions of track paths are opposite between the two recording layers, for example.
Focus leading into the information recorded in each recording layer of the two-layer type optical disc is realized by the control of focus servo. More specifically, if an objective lens in an optical pickup is displaced in a direction of approaching the optical disc, in the case where the L0 layer is focused on, the focus error signal of reflected light from, e.g., a semitransparent reflective film of the L0 layer converges on “0”. Moreover, if the objective lens is raised up and displaced in the direction of approaching the optical disc, in the case where the L1 layer is focused on, the focus error signal of reflected light from a reflective film of the L1 layer converges on “0”. As described above, if the objective lens is displaced in the direction of approaching the optical disc, two focus error signals with an S-shaped waveform (hereinafter referred to as “S-shaped signals”) are obtained, which are centered on the position of the objective lens when the L0 layer is focused on (hereinafter referred to as a “focused focal position” of the L0 layer, as occasion demands) and centered on the focused focal position of the L1 layer. It is possible to focus on each of the L0 layer and the L1 layer, by displacing the position of the objective lens up and down, under the control of focus servo using the two S-shaped signals.
Moreover, as described in a patent document 3, there is also a technology in which management information or the like is recorded only in a certain recording layer out of the plurality of recording layers. Furthermore, as described in a patent document 4, there is also a technology in which the plurality of recording layers are disposed without overlapped each other.    Patent document 1: Japanese Patent Application Laid Open NO. 2000-311346    Patent document 2: Japanese Patent Application Laid Open NO. 2001-23237    Patent document 3: Japanese Patent Application Laid Open NO. 2003-168221    Patent document 4: Japanese Patent Application Laid Open NO. Hei 9-147415